Electrochemical cells, such as prismatic cells and button cells, can be utilized in a variety of electronic devices. Alkaline prismatic cells are well suited for use in devices including wireless devices such as a keyboard or a mouse, MP3 players, flash MP3 players, and BLUETOOTH® wireless headsets. Alkaline button cells, such as zinc-manganese dioxide and zinc-silver cells, are often used in small devices such as watches and hand-held calculators, and zinc-air cells are particularly useful in electronic hearing aids. Commercial alkaline cells typically comprise a negative electrode (anode) including an anode casing and a positive electrode (cathode) including a cathode casing. Both the anode casing and the cathode casing have similarly-shaped bodies such as a pan or a cup, each with a closed end and an open end generally opposite the closed end. The negative electrode typically employs zinc or a zinc alloy as the active material and an alkaline electrolyte, such as potassium hydroxide. The anode casing is inserted into the cathode casing after all of the cell materials are placed at desired locations within the anode and cathode casings, with electrical insulating material therebetween, wherein the cell is sealed generally by crimping. In the past, mercury was utilized in the negative electrode active material mixture to improve electrical conductivity within the negative electrode and to reduce hydrogen gassing which can occur in the cell during discharge, as well as during storage and periods of non-use. Due to concerns regarding the environment and the health of humans and animals, it is desirable to substantially decrease or eliminate mercury from all electrochemical cells, including button cells.
The elimination of mercury without making any other changes to the cell can result in cell leakage caused by one or more of gassing within the cell, capillary action of the sealing areas, electrochemical creepage driven by the potential difference within the cell, and a damaged cell sealing component. Furthermore, pressure within button zinc-air cells can cause delamination of the hydrophobic layer from the air electrode, which creates void space, causing electrolyte to accumulate therein. This accumulation of electrolyte results in a barrier for air to reach the air electrode of the cell.
Numerous different approaches have been taken in an attempt to eliminate the leakage problem in button-type cells, while substantially decreasing or eliminating the mercury content of the cell at the same time.
U.S. Pat. No. 6,830,847 to Ramaswami et al. relates to a zinc-air button cell comprising a cathode casing and an anode casing wherein the anode casing is inserted into the cathode casing. The anode casing is formed of multi-clad metal layers, for example nickel/stainless steel/copper. A reportedly protective metal is plated onto the exposed peripheral edge of the anode casing. The metal is desirably selected from copper, tin, indium, silver, brass, bronze or gold.
U.S. Pat. No. 6,602,629 to Guo et al. discloses an improved button air cell that contains zero mercury, is free of indium on the sealing surface of the anode cup, and has an active material comprising zinc alloyed with lead. Indium or another metal with a higher hydrogen overvoltage can be put onto the interior surface of the anode cup or portions of the inner surface that are not in the seal area.
U.S. Patent Application Publication No. 2003/0211387 to Braunger et al. relates to a galvanic element with an alkaline electrolyte and a zinc negative electrode, in a housing in the form of a button cell, wherein at least the outer layer of the cell's cap is coated with a Cu—Sn-alloy containing no nickel or with a Cu—Sn—Zn-alloy containing no nickel. The same coating can also be applied to the inner surface of the cap, the inner surface of the other half of the cell housing, its cup, and the outer surface thereof is also coated with the same material, if necessary.
U.S. Pat. No. 6,060,196 to Gordon et al. relates to a zinc alloy anode-based electrochemical cell, which generates gases and/or energy. The structure of the cell is such that a zinc alloy anode material is the integral part of housing and is in contact with an alkaline electrolyte containing minor amounts of corrosion inhibitors. The zinc anode cap is a zinc alloy containing at least one metal from the group consisting of lead, indium, cadmium, bismuth, and combinations thereof. The zinc cap has a copper, tin, or stainless steel clad outer layer to reportedly protect the zinc anode from atmospheric corrosion.
Japanese Laid-Open Publication No. 07-057705 to Toshiba Battery Co. Ltd. relates to a battery that uses non-amalgamated zinc as a negative active material and an alkaline electrolyte and has a sealing plate which also serves as a negative current collector inside a positive case. The sealing plate is formed with a copper/stainless steel/nickel three layer clad material, and then part or the whole of the sealing plate is covered with lead, tin, indium, or bismuth, or an alloy by electroless plating or electrolytic plating. It is reported that cracks or pinholes on the surface produced on formation are covered to eliminate an active site for hydrogen gas evolution to retard the hydrogen gas evolution by covering with the metal with high hydrogen over potential.
Japanese Laid-Open Patent Application No. 50-134137 to Toshiba Ray O Vac Co. discloses that the rim part of a nickel plated anode sealing plate, which attaches to the insulator packing of the plate, is coated with nitrogen oxides. The nitrogen oxide treatment reportedly prevents electrolyte leakage.
Various problems have been encountered when plating anode casings by processes such as electrolytic plating or electroless plating, or both. Examples of such problems include anode casing finishes having a hazy, cloudy, matte, or the like appearance which is undesirable and can result in greater amounts of gassing than a more desirable appearance; anode casings having scratches, cuts, or the like that can result in an undesirable appearance and increased gassing if a substrate having a lower hydrogen overvoltage is exposed; nesting of anode casings during the plating process, resulting in incomplete, or non-uniform plating, or a combination thereof; casings sticking to each other, such as between flat surfaces thereof, due to surface tension of the plating solutions or casings floating in the plating solution, and combinations thereof which can cause incomplete or non-uniform plating, or a combination thereof; improper flow of anode casings in a plating device during plating; and casings having damaged areas such as bent and warped areas.